The present invention may be used for an orthogonal frequency division multiplexing (OFDM) communication scheme, a DFT-S-OFDM communication scheme, and an orthogonal frequency division multiple access (OFDMA) communication scheme, and may also be used for a communication scheme which transmits data by a plurality of sub-carriers and maintains orthogonality between the sub-carriers.
Representative communication methods for the multi-carrier system are the OFDMA scheme, the DFT-S-OFDM (DFT Spreading OFDM) scheme, and the OFDMA scheme, and detailed description thereof will hereinafter be described in detail.
OFDM Scheme
The OFDM scheme will hereinafter be described in detail.
According to the basic principles of the OFDM scheme, the OFDM scheme divides a high-rate data stream into many slow-rate data streams, and simultaneously transmits the slow-rate data streams via many carriers. Each of the carriers is called a sub-carrier.
The orthogonality exists between many carriers of the OFDM scheme. Accordingly, although frequency components of the carrier are overlapped with each other, the overlapped frequency components can be detected by a reception end. A high-rate data stream is converted to a low-rate data stream by a serial to parallel (SP) converter. The individual sub-carriers are multiplied by the parallel data streams, the individual data streams are added to the multiplied result, and the added result is transmitted to the reception end.
The several parallel data streams generated by the S/P converter may be transmitted as several sub-carriers to the reception end by an Inverse Discrete Fourier Transform (IDFT). The IDFT scheme can be effectively implemented by an Inverse Fast Fourier Transform (IFFT).
Relative signal dispersion in a time domain occurs by multi-path delay dispersion. Since a symbol duration of the low-rate sub-carrier increases, the temporal signal dispersion decreases. A long guard interval longer than the length of a channel delay spreading is inserted between the OFDM symbols, so that an inter-symbol interference can be reduced. If some parts of the OFDM signals are copied and arranged in the above-mentioned guard interval, the OFDM symbol is cyclically extended to protect the symbol from others.
DFT-S-OFDM Scheme
The DFT-S-OFDM scheme will hereinafter be described in detail. The DFT-S-OFDM scheme is called a single carrier—FDMA (SC-FDMA). The SC-FDMA scheme mainly applied to an uplink performs the spreading based on the DFT matrix in a frequency area before generating the OFDM signal, modulates the spreading result according to the conventional OFDM scheme, and transmits the modulated result.
FIG. 1 is a conceptual diagram illustrating a transmission end based on the DFT-S-OFDM scheme.
Some variables are defined to explain operations of the above-mentioned conventional apparatus of FIG. 1. “N” is indicative of the number of sub-carriers transmitting the OFDM signal. “Nb” is indicative of the number of sub-carriers for a predetermined user. “F” is indicative of a Discrete Fourier Transform (DFT) matrix, “s” is indicative of a data symbol vector, “x” is indicative of a data dispersion vector in the frequency area, and “y” is indicative of an OFDM symbol vector transmitted in the time area.
The SC-FDMA scheme converts each data symbol (s) into a parallel signal by the S/P converter 110. Before the DFT spreading module 120 transmits the data symbol (s), the data symbol (s) is dispersed, as represented by the following equation 1:x=FNb×Nbs  [Equation 1]
In Equation 1, FNb×Nb is indicative of a NB-sized DFT matrix to disperse the data symbol (s).
The sub-carrier mapping unit 130 performs the sub-carrier mapping on the dispersed vector (x) according to a predetermined sub-carrier allocation technique. The mapping resultant signal is converted into a time-domain signal by the IDFT module 140, the time-domain signal is applied to the parallel/serial converter 150, and a desired signal to be transmitted to the reception end is acquired. In this case, the transmission signal applied to the reception end can be represented by the following equation 2:y=FN×N−1x  [Equation 2]
In Equation 2, FN×N−1 is indicative of the N-sized IDFT matrix for converting a frequency-domain signal into a time-domain signal. A cyclic prefix insertion unit 160 inserts a cyclic prefix into the signal (y) created by the above-mentioned method, so that the resultant signal is transmitted. This method capable of generating the transmission signal and transmitting the same to the reception end is called an SC-FDMA method. The size of the DFT matrix can be controlled in various ways to implement a specific purpose.
OFDMA Scheme
The OFDMA scheme will hereinafter be described in detail. The OFDMA scheme is indicative of a multi-access method for providing some parts of the sub-carriers available for a modulation system based on orthogonal sub-carriers to individual users, so that a multi-access between the users is implemented.
The OFDMA provides frequency resources called sub-carriers to the individual users. As well known in the art, the frequency resources are applied to the users independent of each other, so that they are not overlapped with each other.
Data of a user equipment (UE) in the above-mentioned SC-FDMA or OFDMA system can be transmitted to a destination according to the following two methods, i.e., a localized allocation method, and a distributed allocation method.
The localized allocation method is designed to transmit the UE data using resources composed of sub-carriers neighboring with each other.
The distributed allocation method is designed to transmit the UE data using resources composed of sub-carriers spaced apart from each other at intervals of a predetermined distance.
In the meantime, data, pilots, and control information can be transmitted to an uplink. The data transmitted to the uplink is equal to the UE data, and a band allocation or a transport format may be determined by a downlink control signal.
The pilot signal can be classified into a channel quality (CQ) pilot and a demodulation (DM) pilot. The CQ pilot is used to measure a channel quality (CQ). The DM pilot performs channel estimation and data demodulation during the data transmission.
When the user performs the scheduling result from a frequency area at a specific time and transmits the resultant data, the above-mentioned pilot for performing the channel estimation and the data demodulation during the data transmission is transmitted from the corresponding area.
The control information can be classified into two control information, i.e., data-associated control information and non-data-associated control information.
The data-associated control information is required to recover data transmitted from the UE. For example, the data-associated control information may correspond to information associated with a transport format or HARQ-associated information. The amount of data-associated control information is adjusted by the scheduling method of uplink data.
The non-data-associated control information is required to implement downlink transmission of data. For example, the non-data-associated control information may correspond to a channel quality indicator (CQI) capable of generating the ACK/NACK signal for the HARQ operation and performing link adaptation of a downlink.
In the meantime, the format of uplink data (i.e., data transmitted to an uplink), a pilot and control information will hereinafter be described on the basis of the above-mentioned explanation.
FIG. 2 shows an uplink sub-frame structure.
Referring to FIG. 2, the long blocks (LBs) LB#1˜LB#6 contained in the sub-frame structure are used to transmit data and control information. The short block (SBs) SB#1˜SB#6 contained in the sub-frame structure of FIG. 3 are used to transmit the pilot and data.
In the meantime, the CP of FIG. 2 indicates a specific area in which a cyclic prefix is inserted. The format of sub-frames shown in FIG. 2 has been disclosed for only illustrative purposes, and other sub-frame structures may also be applied to the present invention.
As described above, the signals transmitted via the uplink by the user can be combined with each other according to the following three cases:
1) UE data, pilot, and data-associated information
2) UE data, pilot, data-associated control information, and non-data-associated control information
3) Pilot, and non-data-associated control information
The uplink transmission signals for the above-mentioned cases can be multiplexed in various ways.
FIGS. 3 and 4 show the structures in which data and control information are multiplexed and transmitted in an uplink direction.
In more detail, the method of FIG. 3 multiplexes the data-associated control information and the non-data-associated control information along with the UE data, and at the same time multiplexes non-data-associated control information of several UEs.
The method of FIG. 4 multiplexes the data-associated control information and the UE data, but decides a predetermined time-frequency area to transmit non-data-associated control information of several UEs. In more detail, a third resource block of the second sub-frame of FIG. 2 denotes a band allocated to transmit the non-data-associated control information.
In the meantime, if UE data exists in the case of FIG. 4, the non-data-associated control information of the UE is not transmitted to a band prescribed for the non-data-associated control information, and is transmitted to a transmission band of the UE data, so that the SC-FDMA characteristics can be maintained.
The data-associated control information transmitted to the uplink includes the ACK/NACK signal associated with downlink data and CQI information required for the downlink scheduling. Two multiplexing methods are used to transmit the non-data-associated control information, i.e., a TDM scheme and a FDM scheme. The TDM scheme guarantees a characteristic time area to transmit the ACK/NACK and CQI signals. The FDM scheme arranges the ACK/NACK and CQI signals to a specific frequency area.
However, a detailed method for transmitting the above-mentioned ACK/NACK signal has not yet been developed, so that specific information indicating whether repetitive transmission is required according to channel environments affected by UE's location within a cell, and a method for allocating the ACK/NACK signal and the data transmission area within a single transmission time interval (TTI) used for the 3GPP LTE must be newly developed and proposed by associated developers or companies.